MdBook Variables preprocessor

A simple preprocessor for mdbook, that look for variable in double brackets and replace with some value that come from the book.toml

Example

SimpleFile.md: ```

something

a contentent with a variable {{name}} ```

book.toml ```toml

... all the basic detail first and then:

[preprocessor.variables.variables] name= "my wonderful name" ```

The implementation got a lot of inspiration and code from the mdbook links preprocessor an mdbook-plantuml.